1. Field of the Invention
The present invention concerns a wind turbine rotor blade in which a lightning discharge member is provided so as to protrude from the surface of a wind turbine blade, and in particular relates to a wind turbine rotor blade lightning discharger that is designed to suppress noise that the lightning discharge member generates and to a wind turbine generator equipped with the same.
2. Description of Related Art
Standard wind turbine generators are equipped with a wind turbine rotor blade having several wind turbine blades that extend in radial directions centered on a rotor head and have a configuration in which the rotor head is supported, at a shaft thereof, by a nacelle that is supported at the top of a tower so as to be capable of turning horizontally, and a generator disposed inside the nacelle is driven by the rotation of this wind turbine rotor blade to perform electrical power generation.
This kind of wind turbine generator tends to be struck by lightning, particularly on portions of the wind turbine blades, and therefore, a receptor (lightning rod member) serving as a lightning discharger is provided on each wind turbine blade, as disclosed in Japanese Unexamined Patent Application, Publication No. 2010-223148. In this case, the receptor is provided at the tip of the blade, which tends to be struck by lightning the most, and a lightning conductor (down conductor) which extends from the receptor passes through the interior of the wind turbine blade and is earthed to the ground via the nacelle and the tower, so as to guide lightning current that occurs when lightning strikes the receptor into the ground.
By providing such receptors, it is possible to quite reliably prevent lightning striking the tips of the blades; however, when lightning strikes a position shifted to the blade root side by a distance of several meters from the blade tip, the lightning current may destroy the outer skin of the blade and reach the interior of the blade, causing a risk of damage to the wind turbine blade. In future, with the increasing size of wind turbine generators, there will be a trend towards larger and larger blades, and in regions close to the shore, because the lightning attachment rate is lower overall due to the build up of salt compounds on the blade surfaces, there is a trend towards a greater risk of lightning striking parts other than the receptors provided at the tips of the blades.
Thus, numerous diverter strips, which are button-shaped metal pieces, are provided on the blade surface in areas other than the blade tip where the receptor is provided, and by means of these diverter strips, lightning current that occurs when lightning strikes areas other than the blade tip is allowed to flow along the surface of the wind turbine blade via the individual diverter strips and can be guided to the receptor. By doing so, each diverter strip does not need to be provided with a lightning conductor, and therefore, it is possible to improve the lightning resistance of the wind turbine blade with a simple structure.
However, when numerous diverter strips are disposed on the surface of the wind turbine blade as described above, because of the diverter strips which project from the surface of the wind turbine blade, when the wind turbine rotor blade rotates, Karman vortices (or Karman vortex streets) are generated on the downstream side of the diverter strips with respect to the airflow flowing at the surface of the wind turbine blade, and this causes annoying fluid noise. If the diverter strips are embedded so that the diverter strips are flush with the surface of the wind turbine blade, the problem of fluid noise can be eliminated. Doing so, however, reduces the lightning attachment performance of the diverter strips and their ability to transmit the lightning current, and depressions for embedding the diverter strips are inevitably produced in the surface of the wind turbine blade, which leads to force concentration in the periphery of these depressions, thus causing a decrease in strength of the wind turbine blade.